1. Field of the Invention
Generally, the present invention relates to the field of fabricating integrated circuits, and, more particularly, to the monitoring of process flow quality and production yield by evaluating measurement data.
2. Description of the Related Art
Today's global market forces manufacturers of mass products to offer high quality products at a low price. It is thus important to improve yield and process efficiency to minimize production costs. This holds especially true in the field of semiconductor fabrication, since, here, it is essential to combine cutting-edge technology with mass production techniques. It is, therefore, the goal of semiconductor manufacturers to reduce the consumption of raw materials and consumables while at the same time improving process tool utilization. The latter aspect is especially important, since, in modern semiconductor facilities, equipment is required which is extremely cost-intensive and represents the dominant part of the total production costs. Consequently, high tool utilization in combination with a product yield, i.e., with a high ratio of good devices and faulty devices, results in increased profitability.
Integrated circuits are typically manufactured in automated or semi-automated facilities, thereby passing through a large number of process and metrology steps to complete the device. The number and the type of process steps and metrology steps a semiconductor device has to go through depends on the specifics of the semiconductor device to be fabricated. A usual process flow for an integrated circuit may include a plurality of photolithography steps to image a circuit pattern for a specific device layer into a resist layer, which is subsequently patterned to form a resist mask used in further processes for forming device features in the device layer under consideration by, for example, etch, implant, deposition, polish processes and the like. Thus, layer after layer, a plurality of process steps are performed based on a specific lithographic mask set for the various layers of the specified device. For instance, a sophisticated CPU requires several hundred process steps, each of which has to be carried out within specified process margins to fulfill the specifications for the device under consideration. Since many of these processes are very critical, a plurality of metrology steps have to be performed to efficiently control the process flow. Typical metrology processes may include the measurement of layer thickness, the determination of dimensions of critical features, such as the gate length of transistors, the measurement of dopant profiles, the number, the size and the type of defects, electrical characteristics and the like. As the majority of the process margins are device-specific, many of the metrology processes and the actual manufacturing processes are specifically designed for the device under consideration and require specific parameter settings at the adequate metrology and process tools.
In a semiconductor facility, a plurality of different product types are usually manufactured at the same time, such as memory chips of different design and storage capacity, CPUs of different design and operating speed and the like, wherein the number of different product types may even reach hundreds and more in production lines for manufacturing ASICs (application specific ICs). Since each of the different product types may require a specific process flow, different mask sets for the lithography, specific settings in the various process tools, such as deposition tools, etch tools, implantation tools, chemical mechanical polishing (CMP) tools, metrology tools and the like, may be necessary. Consequently, a plurality of different tool parameter settings and product types may be encountered simultaneously in a manufacturing environment, thereby also creating a huge amount of measurement data, since typically the measurement data are categorized in accordance with the product types, process flow specifics and the like.
Hereinafter, the parameter setting for a specific process in a specified process tool or metrology or inspection tool may commonly be referred to as process recipe or simply as recipe. Thus, a large number of different process recipes, even for the same type of process tools, may be required which have to be applied to the process tools at the time the corresponding product types are to be processed in the respective tools. However, the sequence of process recipes performed in process and metrology tools or in functionally combined equipment groups, as well as the recipes themselves, may have to be frequently altered due to fast product changes and highly variable processes involved. As a consequence, the tool performance in terms of throughput and yield are very critical manufacturing parameters as they significantly affect the overall production costs of the individual devices. Therefore, great efforts are made to monitor the process flow in the semiconductor plant with respect to yield affecting processes or process sequences in order to reduce undue processing of defective devices and to identify flaws in process flows and process tools. For example, at many stages of the production process, inspection steps are implemented for monitoring the status of the devices. Moreover, other measurement data may be generated for controlling various processes, in which the measurement data may be used as feed forward and/or feedback data. It turns out, however, that the relevance of the inspection and measurement data may not necessarily be suitable for the evaluation of process quality, or the usability of measurement data for control applications may be reduced. For example, in certain processes, a large number of defects may be created, which may then be efficiently detected by respective inspection techniques, while the relevance of the large number of defects may not be appropriately evaluated with respect to the final operational behavior of the device due to a significant amount of irrelevant contributions. Consequently, implicit information that may be “encoded” in the measurement and inspection data, obtained by elaborate metrology techniques, may possibly be discarded, thereby contributing to a reduced production yield.
In view of the situation described above, there is therefore a need for an enhanced technique that enables efficient use of process data in a semiconductor production process.